There have been developed various piezoelectric elements in which displacement applied from the outside can be converted into electricity or, conversely, applied electricity can be converted into displacement. The applicants of this application have also developed a composite piezoelectric element provided with a piezoelectric ceramic and an organic polymer material containing air bubbles mixed therein as described in Patent Literature 1.
The composite piezoelectric element described in Patent Literature 1 is produced through processing steps shown in FIG. 2(b).
That is, the composite piezoelectric element is produced by first performing grooving to form a plurality of grooves in the ceramic by machining, next performing filling to fill the grooves with a resin which evaporates at a predetermined temperature, then performing foaming to form the organic polymer material having air bubbles mixed therein by heat treatment of the resin at a temperature where the resin evaporates, then performing thickness processing to polish a composite material including the ceramic and the organic polymer material to a necessary thickness, then performing electrode formation processing on the polished surface, i.e., the surface on which an electrode is to be formed, and finally performing polarization processing.
Therefore, the composite piezoelectric element described in Patent Literature 1 includes the organic polymer material containing air bubbles mixed therein and thus has the advantage that acoustic impedance can be decreased while an electromechanical coupling coefficient which indicates the performance of a piezoelectric element is maintained high.
In addition, the composite piezoelectric body is usually expressed by “the number of XYZ directions in which piezoelectric ceramic can be exposed in end surfaces-the number of XYZ directions in which organic polymer material can be exposed in end surfaces”, such as 1-3 type, 2-2 type, 0-3 type, 3-0 type, or the like.